Pneumatic radial tires

Abstract: A pneumatic radial tire for heavy duty vehicles comprises a radial carcass (1), a belt (4), a belt reinforcing layer (5; 5a, 5b) and a tread (6), in which the belt is formed by a zigzag and continuously extending rubberized strip (8) containing one or more cords (7) therein, and the belt reinforcing layer is constituted by arranging a rubberized cord ply in a region corresponding to at least a side region of the tread, and the belt and the belt reinforcing layer satisfy a ratio h/h O of not more than 65%. In the tire of the above structure, the occurrence of shoulder wear is controlled together with the avoidance of belt end separation and the durability is improved.

Description: This invention relates to pneumatic radial tires, and more particularly to a pneumatic heavy duty radial tire for use with trucks and buses under a high internal pressure which avoids separation failure at the belt end by specifying the arrangement of cords constituting the belt. As this type of the tire, JP-Y-48-96259 discloses a tire wherein a belt having no cut surface of cord at its end is formed by bending cords slantly extended with respect to an equatorial plane of the tire at a widthwise end of the belt so as to zigzag extend in a circumferential direction of the tire for avoiding the occurrence of separation failure at the belt end. In such a belt, the cords are crossed with each other at the widthwise end of the belt and continuously extended at a bent state, so that there is caused no cord shifting between upper and lower belt layers at the belt end and hence the stretching of the belt end in the circumferential direction is very small as compared with the conventional belt structure wherein rubberized cord plies having a given width are laminated one upon the other so as to cross the cords of the upper and lower plies with each other. This is particularly conspicuous when using high elastic cords such as steel cord and the like. On the other hand, the distance or diameter from the rotational axis of the tire to the outer surface of a tread is smaller at a widthwise end of the tread than at the widthwise central portion of the tread, i.e. there is a difference in diameter between the tread end and the tread center, so that a shearing deformation in the circumferential direction is apt to be caused at the widthwise end of the tread locating within a ground contact area during running under load in the case of heavy duty pneumatic tires for trucks and buses having a large diameter difference. The shearing deformation creates a large slipping between the tread end surface and the road surface, and consequently bring about a so-called shoulder wear wherein the tread end is prematurely worn as compared with the other portion of the tread. To this end, the belt cord plies are laminated one upon the other so as to cross the cords of the upper and lower plies with each other, whereby stretching of the belt end in the circumferential direction is ensured to absorb the diameter difference between the central portion of the tread in the widthwise direction and the widthwise end thereof. However, when a belt having a very small stretching in the circumferential direction at the belt end is used as mentioned above, since the stretching in the circumferential direction can not be expected at the belt end, it is difficult to absorb the diameter difference between the widthwise central portion and the widthwise end of the tread and hence the occurrence of shoulder wear is not avoided. Such an occurrence of shoulder wear still remains a serious problem in a tire using a belt having no cut surface of cord at the belt end for avoiding belt end separation. As a means for controlling shoulder wear, it is considered to render the contour of the tread surface into a crown shape having a large radius of curvature for reducing the diameter difference between the central portion and the end portion of the tread. In this case, however, the ground contact pressure rises at the tread end to increase the heat build-up and hence there is apt to be caused separation failure at the belt end. A tire according to the preamble of claim 1 is known from EP-A-0 501 782. It is, therefore, an object of the present invention to provide a pneumatic radial tire capable of controlling the occurrence of shoulder wear together with the avoidance of belt end separation. According to the invention, there is provided a pneumatic radial tire comprising a radial carcass toroidally extending between a pair of bead cores, a belt superimposed about the carcass, a belt reinforcing layer laid on the belt, and a tread laid thereon, in which the belt is formed by a zigzag and continuously extending rubberized strip containing one or more cords therein substantially in the circumferential direction of the tread so as to slantly extend with respect to an equatorial plane of the tire from one end to the other end at both side edges of the belt through a bent state at each side edge; and the belt reinforcing layer is constituted by arranging a rubberized cord ply containing a plurality of cords slantly arranged with respect to the equatorial plane in a region corresponding to at least a side region of the tread; and the belt and the belt reinforcing layer satisfy a relationship that a distance from an intersect between a normal line of a rotational axis of the tire passing through an outermost widthwise end of the belt and an outer surface of the tread to a position of maximum tire diameter is not more than 65% of a distance from an intersect between a normal line of a rotational axis of the tire passing through an outermost widthwise end of the belt reinforcing layer and an outer surface of the tread to the position of maximum tire diameter. In a preferred embodiment of the invention, the belt reinforcing layer is comprised of two rubberized cord plies, the cords of which plies being crossed with each other at an inclination cord angle with respect to the equatorial plane larger than that of the belt. The invention will be further described with reference to the accompanying drawings, wherein: Fig. 1 is a diagrammatically sectional view of a first embodiment of a pneumatic radial tire according to the invention; Fig. 2 is a diagrammatically sectional view of a second embodiment of a pneumatic radial tire according to the invention; Fig. 3 is a diagrammatically sectional view of a third embodiment of a pneumatic radial tire according to the invention; Fig. 4 is a diagrammatically sectional view of a fourth embodiment of a pneumatic radial tire according to the invention; Fig. 5 is a developed schematic view of an embodiment of the belt used in the invention; Fig. 6 is a schematic view illustrating the formation of the belt used in the invention; Fig. 7 is a developed schematic view of another embodiment of the belt used in the invention; Fig. 8 is a graph showing a relation between relative position of belt and belt reinforcing layer and amount of shoulder wear generated; and Fig. 9 is a diagrammatically sectional view of a comparative embodiment of a pneumatic radial tire. In Figs. 1 to 4 are shown various embodiments of pneumatic radial tires according to the invention. In these figures, numeral 1 is a radial carcass toroidally extending between a pair of bead cores 2a and 2b. Moreover, the carcass 1 is comprised of one carcass ply in the illustrated embodiments. The carcass 1 is provided about each of the bead cores 2a, 2b with three bead portion reinforcing layers 3a, 3b, two of which layers being turned around the bead core from outside of the tire to inside thereof. Further, a belt 4, a belt reinforcing layer 5 (or 5a and 5b in Figs. 2 and 4) and a tread 6 are laid on the carcass 1 in the radial direction of the tire in this order. As shown in Fig. 5 or 7, the belt 4 is formed by extending a rubberized strip 8 containing plural cords 7 (or a single cord) slantly arranged with respect to an equatorial plane O of the tire from one widthwise end of the belt to the other widthwise end thereof, bending at the other end so as to change the inclination with respect to the equatorial plane O in an opposite direction, extending from the other end to the one end, again bending at the one end so as to change the inclination with respect to the equatorial plane O in an opposite direction, and repeatedly conducting the above procedure to zigzag and continuously extend the strip substantially in the circumferential direction of the tire. In the illustrated embodiments, the resulting belt 4 has a two-layer structure. The belt reinforcing layer 5 (or 5a, 5b) arranged outside the belt 4 is comprised of at least one rubberized cord ply containing plural cords arranged in parallel with each other. In Figs 1 and 2, the belt reinforcing layer is formed by laminating two rubberized cord plies so as to cross the cords of these plies with each other, while the belt reinforcing layer in Figs. 3 and 4 is comprised of a single rubberized cord ply. Moreover, it is important that the belt reinforcing layer 5 (or 5a, 5b) is arranged so as to cover each side portion of the tread including at least a tread end. In the embodiments of Figs. 1 and 3, the belt reinforcing layer 5 is arranged so as to cover a region corresponding to the full width of the tread, while the belt reinforcing layer 5a or 5b is arranged so as to cover only a region corresponding to each side portion of the tread. As the formation of the belt, JP-Y-48-96259 discloses a method wherein a rubberized cord strip is returned 1-4 times by a given belt width in a rotating axial direction of the tire every winding in the circumferential direction of the tire and shifted by a given strip width before or after the wound strip in the circumferential direction every return to form a belt. According to this method, a belt as shown in Fig. 5 can be obtained. However, the above method restricts the inclination cord angle with respect to the equatorial plane of the tire to a narrow range. Therefore, if it is intended to set the inclination cord angle within a favorable range from the viewpoint of performance in accordance with the width and circumferential length of the belt, it is desirable to adjust the inclination cord angle within a wider range. In the latter case, the formation of the belt is carried out by determining a circumferential pitch for shifting a rubberized strip 8 containing one or more cords 7 therein before or after the wound strip in the circumferential direction so that an interval between a first wound strip 8 and next wound strip 8 is rendered into a given length by rotating the tire N times (integral number) every bending M times (integral number) at both belt ends as shown in Fig. 6. In Fig. 6 is shown a state that the strip 8 is started from a position 9 and bent M times at both belt ends during the rotation of the tire N times to arrive at a position 10 adjacent to the starting point 9. According to this method, it is possible to arrange the wound strip at substantially a desired inclination cord angle with respect to the equatorial plane without generating a gap between the adjacent wound strips. The belt obtained by this method is shown in Fig. 7. In this case, a crossing line 11 apparently showing an interweave of the strip 8 is created as shown in Fig. 7. In the conventional pneumatic radial tire, a belt obtained by laminating plural rubberized cord plies each containing a large number of cords therein so as to cross the cords of these plies with each other (hereinafter referred to as a conventional belt) is widely used and has cut surfaces of the cords at each ply end, so that there is a problem that belt end separation is easily caused starting from the cut surface of the cord. As a countermeasure for solving this problem, it is effective to avoid the cut surfaces of the cords resulting in the occurrence of belt end separation by using a belt in which the cords are zigzag extended in the circumferential direction through bending at both belt ends (hereinafter referred to as a zigzag belt). Furthermore, the zigzag belt creates no shifting of cord between upper and lower plies at the belt end and does not lower the belt tension at the belt end different from the conventional belt, so that it provides a hoop effect higher than that of the conventional belt. In the zigzag belt, however, the stretching in the circumferential direction at the belt end is very small, so that it is difficult to absorb the diameter difference between the central portion and the end portion in the widthwise direction of the tread and hence the occurrence of shoulder wear can not be avoided as previously mentioned. The inventors have examined the relation between the crown shape of the tread and the rigidity of the belt on the shoulder wear in detail and acquired the following knowledge. That is, since the stretching in the circumferential direction is very small at the belt end in the zigzag belt, it is desirable that the zigzag belt is arranged in a region having a small diameter difference with respect to the central portion of the tread. Even if the width of the zigzag belt is made relatively narrow as compared with the conventional belt in order to develop the high hoop effect at the belt end, the zigzag belt can still maintain the carcass shape as a role of the belt. Further, when the belt width is made narrow, the rigidity in the circumferential direction at the shoulder portion of the tread can be reduced, whereby it is possible to absorb the diameter difference. However, when the narrow-width zigzag belt is arranged in the tire, both side regions outside the belt (tread rubber supporting the tread end portion) are not restrained by the zigzag belt and the rubber at the tread end easily moves, so that the slipping of the rubber at the tread end portion in the ground contact area increases to again cause a problem of shoulder wear. To this end, a belt reinforcing layer made from a rubberized ply having a given width is arranged between the belt and the tread at a region ranging from at least an end portion of the belt to the tread end portion, whereby the movement of tread rubber at both sides of the belt is restrained to control the occurrence of shoulder wear. As the arrangement of the belt and the belt reinforcing layer for controlling the occurrence of shoulder wear, a distance h from an intersect between a normal line of a rotational axis of the tire passing through an outermost widthwise end of the belt and an outer surface of the tread to a position of maximum tire diameter is not more than 65% of a distance h 0 from an intersect between a normal line of a rotational axis of the tire passing through an outermost widthwise end of the belt reinforcing layer and an outer surface of the tread to the position of maximum tire diameter as shown in Figs. 1 to 4. In this connection, when the ratio of distance h to distance h 0 is varied in the same tire, the amount of shoulder wear generated is measured to obtain results as shown in Fig. 8. As seen from Fig. 8, when the ratio h/h 0 exceeds 65%, the shoulder wear suddenly proceeds. In order to restrain the tread rubber at the tread end portion, it is advantageous that the outermost end of the belt reinforcing layer in the widthwise direction of the belt is located near to the tread end portion, preferably within a range of 85-95% of a distance between both side ends of a ground contact area of the tread. Moreover, the position of the outermost end of the belt may be determined from the outermost end of the belt reinforcing layer as a standard. If the width of the belt is too narrow, the maintenance of the carcass shape may be obstructed, so that it is preferable that the width of the belt is at least 50% of the maximum width of the carcass at radial section. Since the rigidity of the belt is high, it is preferable that the belt reinforcing layer has a two-ply structure of crossing cords of these plies with each other. However, when the belt reinforcing layer is comprised of plural plies, if the inclination cord angle of the belt reinforcing layer with respect to the equatorial plane is small, the diameter difference at the tread end arranging the belt reinforcing layer becomes large and also the deformation of the belt reinforcing layer becomes large, so that the interlaminar shearing strain between the belt and the belt reinforcing layer increases to facilitate the occurrence of separation between the layers. In this connection, it is desirable to reduce the interlaminar shearing strain by making the inclination cord angle of the belt reinforcing layer larger than that of the belt. Since the belt according to the invention has no cut surface of cord resulting in the occurrence of separation failure, even if the inclination cord angle of the belt is small, belt end separation hardly occurs. Therefore, the inclination cord angle of the belt with respect to the equatorial plane is preferably within a range of 10-30°. Moreover, sufficient hoop effect is obtained even if the width of the belt is narrow as previously mentioned. On the other hand, it is not desirable to excessively increase the circumferential rigidity of the belt reinforcing layer from a viewpoint of the reduction of shearing deformation generated at the tread end in addition to the avoidance of interlaminar separation to the belt. In this connection, the inclination cord angle of the belt reinforcing layer is preferably larger by at least 5° than that of the belt or within a range of 20-50° with respect to the equatorial plane of the tire. The following examples are given in illustration of the invention and are not intended as limitations thereof. There are prepared four pneumatic radial tires having a tire size of 11/70 R22.5 according to a structure as shown in Figs. 1-4 and a specification as shown in Table 1. For comparison, there is prepared a comparative tire having the same tire size according to a structure as shown in Fig. 9 and a specification as shown in table 1. Moreover, cords of 3x0.20 mm + 6x0.36 mm are used in the belt and the belt reinforcing layer. Further, the formation of the belt is carried out according to a manner shown in Fig. 6 at M = 9 and N = 4 or a circumferential pitch f = 9/4, whereby a belt as shown in Fig. 7 is obtained. Each of these test tires is mounted onto a rim of 7.5 x 22.5 size and inflated under an internal pressure of 7.5 kgf/cm 2 . Such a tire is mounted onto an idle shaft of a so-called 2D-4 vehicle in which two tires are mounted onto each side of a driving shaft and idle shaft on a rear wheel of the vehicle and run on a paved road under normal load over a distance of 20,000 km, and thereafter a worn state of the tread surface is measured. The measured results are also shown in Table 1, in which a value obtained by dividing an average of maximum worn depth in ribs at both tread ends by a maximum worn depth in a rib at the central portion of the tread is represented by an index of shoulder wear. Fig. 1 Fig. 2 Fig. 3 Fig. 4 Fig. 9 Belt end count (cords/50 mm) 32 32 32 32 32 inclination cord angle (°) ±18 ±18 ±18 ±18 ±18 Belt reinforcing layer end count (cords/50 mm) 25 25 25 25 - inclination cord angle (°) ±30 ±30 ±30 ±30 - h/h 0 x 100(%) 55 55 55 55 - Index of shoulder wear 1.05 0.97 1.25 1.20 2.45 Example Comparative Example As mentioned above, according to the invention, there can be provided pneumatic radial tires realizing control of shoulder wear together with avoidance of belt end separation and having an excellent durability.

Claims: A pneumatic radial tire comprising a radial carcass (1) toroidally extending between a pair of bead cores (2a, 2b), a belt (4) superimposed about the carcass, and a tread (6) laid thereon, the belt (4) being formed by a zigzag and continuously extending rubberized strip (8) containing one or more cords (7) therein substantially in the circumferential direction of the tread so as to slantly extend with respect to an equatorial plane (O) of the tire from one end to the other end at both side edges of the belt through a bent state at each side edge; characterized in that a belt reinforcing layer (5; 5a; 5b) is laid between the belt and the tread, and the belt reinforcing layer (5; 5a, 5b) is constituted by arranging a rubberized cord ply containing a plurality of cords slantly arranged with respect to the equatorial plane in a region corresponding to at least a side region of the tread; and the belt (4) and the belt reinforcing layer (5: 5a, 5b) satisfy a relationship that a distance (h) from an intersect between a normal line of a rotational axis of the tire passing through an outermost widthwise end of the belt and an outer surface of the tread to a position of maximum tire diameter is not more than 65% of a distance (h 0 ) from an intersect between a normal line of a rotational axis of the tire passing through an outermost widthwise end of the belt reinforcing layer and an outer surface of the tread to the position of maximum tire diameter. A pneumatic radial tire as claimed in claim 1, characterized in that the belt reinforcing layer (5; 5a, 5b) is comprised of two rubberized cord plies laminated so as to cross the cords of these plies with each other and has an inclination cord angle larger than that of the belt (4) with respect to the equatorial plane (O). A pneumatic radial tire as claimed in claim 1 or 2, characterized in that an outermost end of the belt reinforcing layer (5; 5a, 5b) in the widthwise direction of the belt is located within a range of 85-95% of a distance between ends of a ground contact area of the tread. A pneumatic radial tire as claimed in any of claims 1 to 3, characterized in that the width of the belt (4) is at least 50% of the maximum width of the carcass (1) at radial section. A pneumatic radial tire as claimed in any of claims 1 to 4, characterized in that the belt (4) has an inclination cord angle of 10-30° with respect to the equatorial plane (O) of the tire. A pneumatic radial tire as claimed in any of claims 1 to 5, characterized in that the belt reinforcing layer (5; 5a, 5b) has an inclination cord angle of 20-50° with respect to the equatorial plane (O) of the tire.

Inventors: KADOTA KUNINOBU, KOSEKI HIROYUKI, TOYODA KENJI, KADOTA, KUNINOBU, KOSEKI, HIROYUKI, TOYODA, KENJI
Applicants: BRIDGESTONE CORP, BRIDGESTONE CORPORATION
Application year: 1994
Main classification: B60C9
IPC codes: B60C9, B60C3, B60C11